1. Field of the Invention
This invention relates generally to the field of measuring the volumetric flow rate of a fluid. More particularly, the invention relates to a method and apparatus for measuring the volumetric flow rate of a fluid in a drilling rig return line in real time.
2. Description of the Prior Art
Well control and blowout prevention has become an important concern in the oil and gas drilling industry for a number of reasons. Well blowouts cause higher drilling costs, possible loss of life, and waste of natural resources. An additional reason for concern is the increasing number of governmental regulations and restrictions being placed on offshore drilling operations partially as a result of recent, much-publicized well control incidents.
A kick can be defined as a well control problem in which the pressure found within the drilled formation is greater than the mud or fluid hydrostatic pressure acting on the borehole or face of the formation. This formation pressure causes fluids to flow from the formation into the well bore. In almost all drilling operations, the operator attempts to maintain a hydrostatic pressure greater than the formation pressure and thus prevent kicks. On occasion however, and for various reasons, the formation pressure exceeds the mud pressure and a kick will occur. Kicks have become even more common due to the present trend of increasing drilling rates by using lighter drilling mud.
Another problem encountered when drilling a well is drilling fluid loss into the formation. This problem, known by the shorthand term, "Lost Circulation", occurs where the drilling fluid is flowing into a subterranean formation through which the borehole passes. Such condition should be detected quickly by a driller to prevent damage to such a formation and excessive loss of the drilling fluid.
A number of kick or lost circulation "indicators" can be observed at the surface before a kick has had time to result in a dangerous blowout or excessive time has elapsed since the beginning of lost circulation. Three of these are:
FLOW RATE CHANGE--An increase in the flow-out or flow rate leaving the well while pumping at a constant rate is one of the primary kick indicators. The increased flow rate is interpreted to mean that the formation is forcing formation fluids into the well bore. A decrease in the flow rate exiting from the well while pumping at a constant rate is an indicator of lost circulation.
FLOWING WELL WITH PUMPS OFF--When the rig pumps are not moving the mud, a continued flow-out from the well indicates that a kick is in progress. An exception to this indicator is when the mud in the drill pipe is considerably heavier than that in the annulus, as in the case of a slug.
PIT VOLUME CHANGE--If the volume of fluid in the pits is not changed as a result of surface controlled actions, an increase in pit volume indicates that a kick is occurring. The fluids entering the well bore as a result of the kick displace an equal volume of mud at the flow line and result in a pit gain. A decrease in pit volume under these conditions indicates lost circulation.
Two of the kick early warning signs described above require measurement of an increase in flow rate from the fluid return line while the other requires measurement of an increase in pit volume. These indicators are difficult to interpret when drilling from a floating drilling vessel because of the heaving and rolling of the drilling vessel in response to wind and waves. Floating drilling vessel heaving and rolling creates fluid return line flow rate changes.
It has been found that the time elapsed between the beginning of a kick deep in the well and its detection at the surface by pit level monitoring is too long to provide sufficient time to bring the well under control, usually by adding weight to the drilling fluid.
Studies have shown that accurate differential flow measurement, of the order of 25 gallons per minute (GPM) of a total of 1200 GPM, provides the earliest possible surface detection of kicks and/or lost circulation. This method requires the use of flow meters of high absolute accuracy under widely varying conditions, for both flow-in and flow-out systems.
Presently, flow-in measurement is based on the number of strokes per minute of triplex mud pumps (see FIG. 1). The flow rate obtained from the pump strokes is then corrected by a volumetric pump efficiency. This pump efficiency can fluctuate between 80% to 95%, accounting for inaccuracies of .+-.10% in the flow-in measurement.
The prior flow-out measurement has usually included a "paddle" system installed in the rig return line. The paddle is a hybrid flow meter based on level and target (force) measurements. The prior art paddle has an uncalibrated accuracy of around 40%. With calibration on the rig site, the "absolute" flow-out measurement is still only accurate to 10% or 15%, due to the basic non-linearity of the device, and due to very poor zero stability of the device. Poor zero stability requires frequent re-calibration.
Using the pump strokes and the paddle measurement for flow-in and flow-out, respectively, the best accuracy for the differential flow over the entire fluid flow range, cannot be better than 25% or 300 GPM (gallons per minute) in 1200 GPM. This is more than 10 times the required accuracy, rendering prior methods of differential flow rate measurement inadequate for desired kick detection.
Electro-magnetic flow meters have also been used but have drawbacks. They do not work in oil based muds (conductivity too low). They require complete modification of the return line. In offshore rigs where the return line is short and space is limited, there is usually no way to install them. They require expensive maintenance to sustain their accuracy.
Other industries have developed flow measuring systems suitable for use in waste water monitoring systems where sewer outflows must be monitored for pollution control purposes. These systems obtain flow measurements based on the velocity of the fluid in a channel and the area of the channel occupied by the flowing liquid. Ultrasonic level detectors and Doppler type velocity detection units have been used for these applications.
U.S. Pat. No. 4,217,777 to Newman issued Aug. 19, 1980 discloses such a system and is incorporated herein for essential material and for all other purposes. Also U.S. Pat. No. 4,202,211 to Perry issued May 13, 1980 discloses a similar system and is incorporated herein for essential material and for all other purposes.
Ultrasonic level detection systems are known in the art. Such systems are described in U.S. Pat. No. 4,024,766 to Perry issued May 24, 1977 and in U.S. Pat. No. 4,145,914 to Newman issued Mar. 27, 1979, both of which are incorporated herein for all purposes.
The fluid flow in the rig return line is characterized as supercritical flow. None of the known prior art accurately measures volumetric flow rate for flows in supercritical flow conditions. Though the above four U.S. Patents disclose ultrasonic measuring devices, they do not disclose or teach distinguishing between flow regimes which include laminar and turbulent flow so as to determine sufficiently accurately a volumetric fluid flow rate which can be used for real time kick or lost circulation detection on a drilling rig.
3. Identification of Objects of the Invention
It is an object of the invention to provide an improved method and apparatus for the measurement of volumetric fluid flow rate which is significantly more accurate than prior art methods and provides the measurement in real time while drilling.
It is another object of the invention to provide an improved fluid flow rate measuring system which obtains volumetric flow measurements based on the measured velocity of the fluid in a conduit, the cross-sectional area of the conduit occupied by the flowing fluid, and a determination as to whether the flow is laminar or turbulent.
It is a still further object of the invention to provide an improved system for accurately measuring flow rate by distinguishing between flow regimes.
It is still another object of the invention to provide an improved method and apparatus for use in a return line of a drilling rig to quickly and accurately detect a kick or lost circulation in the well bore.
It is another object of the invention to provide a volumetric flow measuring system for accurately measuring flow rate of a fluid in supercritical flow conditions.
It is still another object of the invention to provide all the foregoing measurements on a floating rig while simultaneously suitably compensating for the effects of rig heave.